arm-toolchain/layer.conf: remove BB_DANGLINGAPPENDS_WARNONLY

This appears to be historical from when the toolchain was in meta-linaro.

It isn't needed anymore, there's one bbappend in meta-arm-toolchain for
grub which is part of oe-core, so will never be dangling.

This variable has a global effect, so leaving it in here has a negative
impact on users.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>

meta-arm-toolchain/conf/layer.conf

@@ -14,8 +14,5 @@ LICENSE_PATH += "${LAYERDIR}/custom-licenses"
 LAYERDEPENDS_arm-toolchain = "core"
 LAYERSERIES_COMPAT_arm-toolchain = "zeus dunfell"
 
-# do not error out on bbappends for missing recipes
-BB_DANGLINGAPPENDS_WARNONLY = "true"
-
 # These variables are used for armcompiler license
 BB_HASHBASE_WHITELIST_append = " ARM_TOOL_VARIANT ARMLMD_LICENSE_FILE LM_LICENSE_FILE"